As an approach to development of third generation DNA sequencers, a technique using a nanopore has been studied. Specifically, the technique is as follows. A hole having a size equivalent to that of DNA (nanopore) is provided in a thin film membrane, and the chambers above and below the thin film membrane are filled with an aqueous solution. Electrodes are provided in the chambers in such a manner that the electrodes are in contact with the aqueous solution, and DNA to be measured is supplied to one of the chambers. The DNA is moved by electrophoresis caused by a potential difference applied between the electrodes in the chambers so that the DNA passes through the nanopore. By measuring the change with time in the ion current flowing between the electrodes, the structural characteristics of the DNA and the nucleotide sequence are determined. This technique is effective in obtaining the structural characteristics of not only DNA but also of various biomolecules.
To produce a nanopore device, methods using a semiconductor substrate, a semiconductor material and a semiconductor process draw attention due to the high mechanical strength and the like. For example, a thin film membrane can be formed using a silicon nitride film (SiN film). A nanopore can be formed because a small pinhole can be created in a membrane by applying voltage stress to the membrane in an aqueous ionic solution and causing dielectric breakdown (NPL 1). In another method, a nanopore can be formed by etching a membrane using the condensed electron beam.
One of the important factors that determine the accuracy in reading DNA with a nanopore sequencer is the thickness of the membrane. That is, the thinner the membrane is, the better it is. This is because the distance between neighboring bases of the four kinds aligning in a DNA chain is approximately 0.34 nm, and more bases enter the nanopore at the same time as the membrane becomes thicker as compared to the distance. As a result, the signal obtained by measuring the current becomes the signal derived from more than one base. Thus, the accuracy in determining the nucleotide sequence deteriorates, and the analysis of the signal becomes more complex.